The heart is generally divided into four chambers, two atrial chambers and the two ventricular chambers. As the heart beats, the atrial chambers and the ventricular chambers of the heart go through a cardiac cycle. The cardiac cycle consists of one complete sequence of contraction and relaxation of the chambers of the heart. The terms systole and diastole are used to describe the contraction and relaxation phases the chambers of the heart experience during a cardiac cycle. In systole, the ventricular muscle cells are contracting to pump blood through the circulatory system. During diastole, the ventricular muscle cells relax, causing blood from the atrial chambers to fill the ventricular chambers. After the period of diastolic filling, the systolic phase of a new cardiac cycle is initiated.
Control over the timing and order of the atrial and ventricular contractions during the cardiac cycle is critical for the heart to pump blood efficiently. Efficient pumping action of the heart requires precise coordination of the contraction of individual cardiac muscle cells. Contraction of each cell is triggered when an electrical excitatory impulse (an “action potential”) sweeps over the heart. Proper coordination of the contractual activity of the individual cardiac muscle cells is achieved primarily by the conduction of the action potential from one cell to the next by gap junctions that connect all cells of the heart into a functional system. In addition, muscle cells in certain areas of the heart are specifically adapted to control the frequency of cardiac excitation, the pathway of conduction and the rate of impulse propagation through various regions of the heart. The major components of this specialized excitation and conduction system include the sinoatrial node (SA node), the atrioventricular node (AV node), the bundle of His, and specialized cells called Purkinje fibers.
The SA node is located at the junction of the superior vena cava and the right atrium. Specialized atrium muscle cells of the SA node spontaneously generate action potentials which are then propagated through the rest of the heart to cause cardiac contraction. This SA node region normally acts as the intrinsic cardiac pacemaker. The action potential generated by the SA node spreads through the atrial wall, causing the atrial chambers to contract and the P-wave of an electrocardiogram signal.
The AV node consists of small, specialized cells located in the lower portion of the atrial chamber. The AV node acts like a bridge for the action potential to cross over into the ventricular chamber of the heart. Once the action potential has crossed over to the ventricular chambers, the bundle of His carries the action potential to specialized cardiac fibers called Purkinje fibers. The Purkinje fibers then distribute the action potential throughout the ventricular chamber of the heart. This results in rapid, very nearly simultaneous excitation of all ventricular muscle cells. The conduction of the action potential through the AV node and into the ventricular chambers creates the QRS-complex of an electrogram signal.
During the cardiac cycle, the action potential moves in an antegrade direction, first causing the atrial chambers to contract and then causing the ventricle chambers to contract. When this occurs the depolarization of the atria is “associated” with the depolarization of the ventricle. However, there are cardiac conditions in which the depolarizations (i.e., contractions) occurring in one chamber of the heart are not associated with subsequent contractions occurring in another chamber of the heart. In these situations, the contractions of these regions of the heart are “disassociated.”
The ability to identify and classify the cardiac depolarizations occurring during a cardiac episode, such as a tachycardia episode, as either associated and disassociated is important for directing any additional analysis of the cardiac episode and for directing the appropriate therapy to treat the cardiac episode. One situation where classifying atrial and ventricular contractions of a tachycardia episode as being either associated or disassociated is in the discrimination, or classification, of ventricular tachycardia episodes from supraventricular tachycardia episodes. The ability to accurately classify a ventricular tachycardia episode from a supraventricular tachycardia episode allows the mechanism of the tachycardia episode to be identified which helps greatly in directing appropriate therapy. A need, however, still exists for a reliable way of classifying the cardiac depolarizations occurring during cardiac episodes as either associated or disassociated.